Differential mobility analysis is a technique for measuring the size distribution of submicron particles i.e., particles smaller than 1.0 .mu.m in diameter. Particles in an aerosol sample are first charged, preferably with a single electron charge, by using a charging device. The charged particles are then fed into an external electrical field between two parallel electrode plates by an accompanying gas flow. The direction of the gas flow is substantially parallel to the electrode plates. Hence, the charged aerosol particles experience two substantially perpendicular motions, one in a direction perpendicular to the electrode plates due to the force by the electrical field and another in a direction parallel to the electrode plates due to the "dragging" of the gas flow. Therefore, the particles can be classified according to their mobilities or migration velocities within discrete mobility ranges by sampling these particles after traveling a specific distance along the gas flow. This mobility classification allows for determination of the size of the classified particles if each particle is charged with only one elementary charge. Differential mobility analysis technique is known in the art. See, for example, Knutson and Whitby, in "aerosol classification by electrical mobility", J. Aerosol Sci., vol.6, p.453, 1975, which is incorporated herein by reference.
A typical differential mobility analyzer ("DMA") has a configuration of two electrodes with opposite electrical potentials. A small aerosol flow is introduced from an aerosol inlet near a first electrode and a larger particle-free sheath flow is simultaneously introduced from a sheath inlet to separate the aerosol flow from reaching a second electrode. An electrical potential drives particles of appropriate polarity across the sheath flow toward the opposite electrode. At a location downstream from the aerosol inlet, a portion of the sheath flow, which is usually small and referred to as monodisperse flow, is extracted while the remaining flow is discharged to an exhaust of the instrument ("excess flow"). Specifically, particles that migrate within a narrow range of velocities are included in the classified aerosol sample flow ("monodisperse"). Particles with higher migration velocities deposit on the second electrode while those with lower migration velocities are discharged with the exhaust flow.
Next, the classified aerosol particles are transported to a detector for counting. Any of a number of particle counting techniques may be used. For example, optical particle sizing can provide some information on relatively large particles, i.e., from 1.0 .mu.m to as small as 0.1 or 0.05 .mu.m in diameter. Information on the refractive index of the particulate material and the particle morphology is needed to interpret the measured data.
For submicron particles, the optical detection may be inefficient. Condensation nucleus counters (CNCs) have been developed to detect small size particles. In a CNC, small particles are grown by vapor condensation. Since a CNC can achieve high counting efficiency, rapid response, and continuous-flow, it has been widely used in combination with a DMA in an aerosol detection system.
FIG. 1 shows functional blocks of a typical aerosol detection system 100 based on differential mobility analysis. Three basic elements are shown: a charging device 110, a DMA 120, and a particle counter 130.
The particle size is inferred from the migration velocity based on the relationship between particle size and the electrical mobility of the particles. This is described by Flagan and Seinfeld in "Fundamentals of Air Pollution Engineering", Prentice-Hall, 1988. The electrical mobility, Z, is defined as the ratio of the migration velocity .upsilon..sub.m to the strength of the applied electrical field, E, ##EQU1## For spherical particles carrying .nu. electrical charges, the mobility Z can be written as ##EQU2## where .mu. is the gas viscosity, D.sub.p is the particle diameter, C.sub.c is an empirically-determined slip correction factor that accounts for noncontinuous aerodynamic effects, and e is the elementary unit of change. Noncontinuous aerodynamic effects may become important if the particle diameter is comparable to or smaller than the mean-free-path .lambda. of the gas molecules. One commonly employed form for this slip correction factor C.sub.c is ##EQU3## where ##EQU4## is known as Knudsen number.
Under typical operating conditions in most practical systems, only a fraction of the particles are charged, and a majority of those charged particles carry single charge, i.e., .nu.=1. Most mobility classifications are performed with positively charged particles. However, negatively charged particles may also be used.
The migration velocity required for a particle to be transmitted from the aerosol inlet flow to the classified aerosol outlet flow of the differential mobility analyzers depends on the geometry of the classifier and on the four flow rates, i.e., an input sample flow rate, an input sheath flow rate, an output classified sample flow rate, and an output excess flow rate. The size of the particles to be classified is selected by adjusting the voltage such that particles with the mobility of particles of the desired size will migrate at the velocity required for transmission. The size distribution of the aerosol is determined by making measurements of the concentrations at a number of sizes spanning the size range of interest.
One critical parameter of a DMA is the size resolution, which is defined as the ratio of the mobility at which the transmission efficiency is the highest to the full width at the half value of the maximum of the DMA transfer function. A number of factors may affect the DMA resolution, including the voltage on the DMA, the diffusion of the particles as they migrate through the DMA, the particle losses to the walls of the DMA, the percentage of the particles with two or more elementary charges in the charged aerosol flow, and others.
Differential mobility analysis has traditionally been performed by making a sequence of measurements at different electric field strengths, i.e., at different voltages applied across the two electrodes of the classifier. Although this method is effective, it is usually slow. Depending on the size range and the desired resolution, the operation time may range from several minutes to more than an hour to measure a size distribution. Wang and Flagan accelerated the measurements dramatically by exponentially ramping the voltage and counting the particles continuously, thereby eliminating the delays between successive measurements that are needed to ensure representative data at each mobility with the stepping-mode of differential mobility analysis. A complete size distribution can be measured in less than one minute with this accelerated scanning-mode of differential mobility analysis. See, Wang and Flagan, "Scanning electrical mobility spectrometer" in Aerosol Sci. Technol., 13, pp.230-240, 1990.
A fully automated version of the scanning mode differential mobility analyzer system has been developed for use aboard the University of Washington C-131 during the Monterey Area Ship Track experiment. See, Russell et al., U.S. patent application Ser. No. 08/730,037, "Automated Mobility-Classified-Aerosol Detector". That instrument employed a radial differential mobility analyzer ("RDMA") in a feedback controlled system that maintained stable flows through continuous pressure variations as the aircraft altitude varied and made size distribution measurements in about 45 s. An enhanced condensation nucleus counter, produced based upon one of the commercial TSI models through collaboration with TSI, and the high transmission efficiency of the RDMA extended the sizing range of this instrument below 5 nm. The integrated system, which was termed the Radial Automated Classified Aerosol Detector (RCAD) incorporated a double bag sampler so that size distributions could be made on air drawn from discrete locations separated in flight time by only 45 s.